Nanowire preparation methods, compositions, and articles

ABSTRACT

Methods of preparing nanowires by reducing metal cations are disclosed and claimed, where the metal cation reduction occurs in at least two stages. Such methods can exhibit improved reproducibility and reduced variability. The product nanowires are useful in, for example, electronics applications.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Patent Application No. 61/723,942, filed Nov. 8, 2012, entitled NANOWIRE PREPARATION METHODS, COMPOSITIONS, AND ARTICLES, which is hereby incorporated by reference in its entirety.

BACKGROUND

The general preparation of silver nanowires (AgNW, 10-200 aspect ratio) from silver ions is known. See, for example, Y. Xia, et al., Angew. Chem. Int. Ed. 2009, 48, 60, and J. Jiu, et al., Mat. Chem. & Phys., 2009, 114, 333, each of which is hereby incorporated by reference in its entirety. These include the “polyol” process, in which a silver salt is heated in a polyol (typically ethylene glycol (EG)) in the presence of polyvinylpyrrolidinone (PVP, also known as polyvinylpyrrolidone), yielding a suspension of AgNW in EG, from which the wires can be isolated and/or purified as desired.

Methods of preparing silver nanowires from silver salts are known, where the salt is added to the reaction mixture in more than one step. See, for example, U.S. Pat. No. 8,052,773, US patent application publication 2011/0174190, Chinese patent application publication 1740405A, and Chinese patent 100342064C.

US patent application publication 2012/0063948A discloses reduction of silver nitrate in the presence of ammonium chloride.

SUMMARY

Some embodiments provide methods to prepare silver nanowires comprising at least two stages.

At least a first stage provides a first composition comprising 1,2-propylene glycol, polyvinylpyrrolidone (PVP), and ammonium chloride. The PVP has weight average molecular weight greater than about 15,000 g/mol, or between about 40,000 and about 60,000 g/mol, or about 50,000 g/mol. The first composition is preferably provided at a temperature less than about 140° C., or between about 80° C. and about 120° C., or about 90° C.

In at least one second stage, a second composition comprising 1,2-propylene glycol and silver nitrate is added to the first composition over a time period that is at least about 16 hrs, or between about 20 hrs and about 28 hrs, or about 24 hrs, during which time at least some of the silver nitrate is reduced to silver nanostructures, such as, for example, silver nanowires.

These and other embodiments may be understood from the description, exemplary embodiments, examples, and claims that follow.

DESCRIPTION

All publications, patents, and patent documents referred to in this document are incorporated by reference herein in there entirety, as though individually incorporated by reference.

U.S. Patent Application No. 61/723,942, filed Nov. 8, 2012, entitled NANOWIRE PREPARATION METHODS, COMPOSITIONS, AND ARTICLES, is hereby incorporated by reference in its entirety.

Introduction

Silver nanowires (AgNW) are a unique and useful wire-like form of the metal in which the two short dimensions (the thickness dimensions) are less than 300 nm, while the third dimension (the length dimension) is greater than 1 micron, preferably greater than 10 microns, and the aspect ratio (ratio of the length dimension to the larger of the two thickness dimensions) is greater than five. They are being examined as conductors in electronic devices or as elements in optical devices, among other possible uses.

A number of procedures have been presented for the preparation of AgNW. See, for example, Y. Xia, et al. (Angew. Chem. Int. Ed. 2009, 48, 60), which is hereby incorporated by reference in its entirety. These include the “polyol” process, in which a silver salt is heated in a polyol (typically ethylene glycol (EG)) in the presence of polyvinylpyrrolidinone (PVP, also known as polyvinylpyrrolidone), yielding a suspension of AgNW in EG, from which the wires can be isolated and/or purified as desired.

Reducible Metal Ions and Metal Products

Some embodiments provide methods comprising reducing at least one reducible metal ion to at least one metal. A reducible metal ion is a cation that is capable of being reduced to a metal under some set of reaction conditions. In such methods, the at least one first reducible metal ion may, for example, comprise at least one coinage metal ion. A coinage metal ion is an ion of one of the coinage metals, which include copper, silver, and gold. Or such a reducible metal ion may, for example, comprise at least one ion of an IUPAC Group 11 element. An exemplary reducible metal ion is a silver cation. Such reducible metal ions may, in some cases, be provided as salts. For example, silver cations might, in some cases, be provided as silver nitrate.

In such embodiments, the at least one metal is that metal to which the at least one reducible metal ion is capable of being reduced. For example, silver would be the metal to which a silver cation would be capable of being reduced.

Preparation Methods and Materials

A common method of preparing nanostructures, such as, for example, nanowires, is the “polyol” process. Such a process is described in, for example, Angew. Chem. Int. Ed. 2009, 48, 60, Y. Xia, Y. Xiong, B. Lim, S. E. Skrabalak, which is hereby incorporated by reference in its entirety. Such processes typically reduce a metal cation, such as, for example, a silver cation, to the desired metal nanostructure product, such as, for example, a silver nanowire. Applicants have observed that reproducibility can be improved and variability reduced if such metal cation reduction is carried out in at least two stages.

A first stage or stages provides a first composition comprising 1,2-propylene glycol, polyvinylpyrrolidone (PVP), and ammonium chloride. The

PVP has weight average molecular weight greater than about 15,000 g/mol, or between about 40,000 and about 60,000 g/mol, or about 50,000 g/mol. The first composition is preferably provided at a temperature less than about 140° C., or between about 80° C. and about 120° C., or about 90° C.

In some embodiments, the components of the first composition are contacted with each other prior to heating. In some embodiments, the first stage or stages may provide the first composition in a series of sub-stages each providing some of the components of the composition. Some of the components may be provided in more than one sub-stage.

In at least some embodiments, the first composition may further comprise silver nitrate. For example, a first portion of the first composition may be provided comprising silver nitrate, followed by a second portion of the first composition comprising ammonium chloride.

In a second stage or stages, a second composition comprising 1,2-propylene glycol and silver nitrate is added to the first composition over a time period that is at least about 16 hrs, or between about 20 hrs and about 28 hrs, or about 24 hrs, during which time at least some of the silver nitrate is reduced to silver nanostructures, such as, for example, silver nanowires.

Nanostructures and Nanowires

In some embodiments, the metal product formed by such methods is a nanostructure, such as, for example, a one-dimensional nanostructure.

Nanostructures are structures having at least one “nanoscale” dimension less than 300 nm, and at least one other dimension being much larger than the nanoscale dimension, such as, for example, at least about 10 or at least about 100 or at least about 200 or at least about 1000 times larger. Examples of such nanostructures are nanorods, nanowires, nanotubes, nanopyramids, nanoprisms, nanoplates, and the like. “One-dimensional” nanostructures have one dimension that is much larger than the other two dimensions, such as, for example, at least about 10 or at least about 100 or at least about 200 or at least about 1000 times larger.

Such one-dimensional nanostructures may, in some cases, comprise nanowires. Nanowires are one-dimensional nanostructures in which the two short dimensions (the thickness dimensions) are less than 300 nm, preferably less than 100 nm, while the third dimension (the length dimension) is greater than 1 micron, preferably greater than 10 microns, and the aspect ratio (ratio of the length dimension to the larger of the two thickness dimensions) is greater than five. Nanowires are being employed as conductors in electronic devices or as elements in optical devices, among other possible uses. Silver nanowires are preferred in some such applications.

Such methods may be used to prepare nanostructures other than nanowires, such as, for example, nanocubes, nanorods, nanopyramids, nanotubes, and the like. Nanowires and other nanostructure products may be incorporated into articles, such as, for example, electronic displays, touch screens, portable telephones, cellular telephones, computer displays, laptop computers, tablet computers, point-of-purchase kiosks, music players, televisions, electronic games, electronic book readers, transparent electrodes, solar cells, light emitting diodes, other electronic devices, medical imaging devices, medical imaging media, and the like.

EXEMPLARY EMBODIMENTS

U.S. Patent Application No. 61/723,942, filed Nov.8, 2012, entitled NANOWIRE PREPARATION METHODS, COMPOSITIONS, AND ARTICLES, which is hereby incorporated by reference in its entirety, disclosed the following twelve non-limiting exemplary embodiments:

A. A method comprising:

providing at least one first composition comprising 1,2-propylene glycol, polyvinyl pyrrolidone, and ammonium chloride;

adding at least one second composition to the at least one first composition, the adding occurring over the course of at least about 16 hrs, and the at least one second composition comprising 1,2-propylene glycol and silver nitrate; and

reducing at least a portion of the silver nitrate to silver nanowires.

B. The method according to embodiment A, further comprising heating the at least one first composition to a temperature less than about 140° C.

C. The method according to embodiment A, further comprising heating the at least one first composition to a temperature between about 80° C. and about 120° C.

D. The method according to embodiment A, further comprising heating the at least one first composition to a temperature of about 90° C. E. The method according to embodiment A, wherein the at least one first composition further comprises silver nitrate. F. The method according to embodiment E, wherein providing the at least one first composition comprises:

providing at least one third composition comprising silver nitrate; and adding at least one fourth composition to the at least one third composition, the at least one fourth composition comprising ammonium chloride.

G. The method according to embodiment E, wherein providing the at least one first composition comprises:

providing at least one third composition comprising polyvinyl pyrrolidone;

forming at least one fourth composition by adding at least one fifth composition to the at least one third composition, the at least one fifth composition comprising silver nitrate; and

adding at least one sixth composition to the at least one fourth composition, the at least one sixth composition comprising ammonium chloride.

H. The method according to embodiment G, further comprising heating the at least one third composition to a temperature less than about 140° C. J. The method according to embodiment G, further comprising heating the at least one third composition to a temperature between about 80° C. and about 120° C. K. The method according to embodiment G, further comprising heating the at least one third composition to a temperature of about 90° C. L. The method according to embodiment A, wherein the adding the at least one second composition to the at least one first composition occurs over the course of between about 20 hrs and about 28 hrs. M. The method according to embodiment A, wherein the adding the at least one second composition to the at least one first composition occurs over the course of about 24 hrs.

EXAMPLES Example 1

To a 500 mL reaction vessel was charged 430 mL propylene glycol, 7.2 g of polyvinylpyrrolidone (50,000 weight average molecular weight), and 2 mL of a 1 wt % solution of ammonium chloride in propylene glycol. The mixture was stirred under nitrogen until solids were in solution, followed by heating to 90° C. A freshly prepared solution of 6 g AgNO₃ in 36 mL propylene glycol was added dropwise over 64 h. After quenching in an ice bath, the product was isolated by settling and centrifugation to give silver nanowires with an average diameter of 40.23 nm and average length of 30.9 μm, based on measurement of at least 100 nanowires.

Example 2 (Comparative)

To a 500 mL reaction vessel was charged 430 mL propylene glycol and 7.2 g of polyvinylpyrrolidone (50,000 weight average molecular weight). The mixture was stirred under nitrogen until solids were in solution, followed by heating to 90° C. To the mixture was added 0.2 mL of a solution of 6 g AgNO₃ in 36 mL propylene glycol, followed by 1.14 mL of a 10 wt % solution of tetrabutylammonium chloride in propylene glycol. After these additions, 35.8 mL of a solution of 6 g AgNO₃ in 36 mL propylene glycol was added to the mixture.

This mixture was stirred at temperature for 24 hr. After quenching in an ice bath, the product was isolated by settling and centrifugation to give silver nanowires with an average diameter of 60 nm and average length of 15.7 μm, based on measurement of at least 100 nanowires.

Example 3

The procedure of Example 1 was repeated, changing the time period for AgNO₃ solution addition from 64 h to 24 h. The resulting silver nanowires had an average diameter of 44.11 nm and average length of 16.7 μm, based on measurement of at least 100 nanowires.

Example 4

The procedure of Example 1 was repeated, changing the time period for AgNO₃ solution addition from 64 h to 67.3 h, and changing the reaction temperature from 90° C. to 75° C. The resulting silver nanowires had an average diameter of 57.2 nm and average length of 20.5 μm, based on measurement of at least 100 nanowires.

Example 5

To a reaction vessel was charged 2000 mL propylene glycol and 33.5 g of polyvinylpyrrolidone (PVP, 50,000 weight average molecular weight).

The mixture was stirred with nitrogen sparging until the PVP dissolved. The mixture was then heated to 91° C. A freshly prepared solution of 28.08 g AgNO₃ in 168 mL propylene glycol was pumped into the reaction vessel at a rate of 0.949 mL/min for 5 min. The silver nitrate pump was then stopped for 5 min. The silver nitrate pump was then restarted at a rate of 34.6 mL/hr and was allowed to run for 30.6 hr. At the time the silver nitrate pump was restarted, 9.4 mL of a 1 wt % solution of ammonium chloride in propylene glycol was pumped into the reaction mixture at a rate of 34.6 mL/hr. The reaction mixture was held for 25 min after the silver nitrate pump was shut off. The reaction vessel was then allowed to cool to room temperature. The resulting silver nanowires had an average diameter of 43.8 nm and average length of 22.9 μm, based on measurement of at least 100 nanowires.

Example 6 To a reaction vessel was charged 8000 mL propylene glycol and 134.0 g of polyvinylpyrrolidone (PVP, 50,000 weight average molecular weight). The mixture was stirred with nitrogen sparging until the PVP dissolved. The mixture was then heated to 91° C. A freshly prepared solution of 112.32 g AgNO₃ in 672 mL propylene glycol was pumped into the reaction vessel at a rate of 0.44 mL/min for 5 min. The silver nitrate pump was then stopped for 5 min.

The silver nitrate pump was then restarted and was allowed to run for 26.7 hr. At the time the silver nitrate pump was restarted, 37.6 mL of a 1 wt % solution of ammonium chloride in propylene glycol was pumped into the reaction mixture at a rate of 150 mL/hr. The reaction mixture was held for 66 min after the silver nitrate pump was shut off. The reaction vessel was then allowed to cool to room temperature. The resulting silver nanowires had an average diameter of 39.8 nm and average length of 17.6 μm, based on measurement of at least 100 nanowires.

The invention has been described in detail with particular reference to a presently preferred embodiment, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims, and all changes that come within the meaning and range of equivalents thereof are intended to be embraced therein. 

What is claimed:
 1. A method comprising: providing at least one first composition comprising 1,2-propylene glycol, polyvinyl pyrrolidone, and ammonium chloride; adding at least one second composition to the at least one first composition, the adding occurring over the course of at least about 16 hrs, and the at least one second composition comprising 1,2-propylene glycol and silver nitrate; and reducing at least a portion of the silver nitrate to silver nanowires.
 2. The method according to claim 1, further comprising heating the at least one first composition to a temperature less than about 140° C.
 3. The method according to claim 1, further comprising heating the at least one first composition to a temperature between about 80° C. and about 120° C.
 4. The method according to claim 1, further comprising heating the at least one first composition to a temperature of about 90° C.
 5. The method according to claim 1, wherein the at least one first composition further comprises silver nitrate.
 6. The method according to claim 5, wherein providing the at least one first composition comprises: providing at least one third composition comprising silver nitrate; and adding at least one fourth composition to the at least one third composition, the at least one fourth composition comprising ammonium chloride.
 7. The method according to claim 5, wherein providing the at least one first composition comprises: providing at least one third composition comprising polyvinyl pyrrolidone; forming at least one fourth composition by adding at least one fifth composition to the at least one third composition, the at least one fifth composition comprising silver nitrate; and adding at least one sixth composition to the at least one fourth composition, the at least one sixth composition comprising ammonium chloride.
 8. The method according to claim 7, further comprising heating the at least one third composition to a temperature less than about 140° C.
 9. The method according to claim 7, further comprising heating the at least one third composition to a temperature between about 80° C. and about 120° C.
 10. The method according to claim 7, further comprising heating the at least one third composition to a temperature of about 90° C.
 11. The method according to claim 1, wherein the adding the at least one second composition to the at least one first composition occurs over the course of between about 20 hrs and about 28 hrs.
 12. The method according to claim 1, wherein the adding the at least one second composition to the at least one first composition occurs over the course of about 24 hrs. 